1. Field of the Invention
The present invention relates to the papermaking arts. More specifically, the present invention relates to the papermaker's fabrics used on the dryer section of a paper machine, and particularly on a single-run dryer section. Such fabrics are commonly referred to as dryer fabrics.
2. Description of the Prior Art
As is well known to those of ordinary skill in the art, the papermaking process begins with the deposition of a fibrous slurry, that is, an aqueous dispersion of cellulosic fibers, onto a moving forming fabric in the forming section of a paper machine. A large amount of water is drained from the slurry through the forming fabric during this process, leaving a fibrous web on its surface.
The newly formed web proceeds from the forming section to a press section, which includes a series of press nips. The fibrous web passes through the press nips supported by a press fabric, or, as is often the case, between two press fabrics. In the press nips, the fibrous web is subjected to compressive forces which squeeze water therefrom, and which adhere its constituent fibers to one another to turn the fibrous web into a sheet. The water squeezed from the web is accepted by the press fabric or fabrics, and, ideally, does not return to the web.
The web, now a sheet, finally proceeds to a dryer section, which includes at least one series of rotatable dryer drums or cylinders, which are internally heated by steam. The sheet itself is directed in a serpentine path sequentially around each in the series of drums by a dryer fabric, which holds the web closely against the surfaces of at least some of the drums. The heated drums reduce the water content of the sheet to a desirable level through evaporation.
It should be appreciated that the forming, press and dryer fabrics all take the form of endless loops on the paper machine and function in the manner of conveyors. It should further be appreciated that paper manufacture is a continuous process which proceeds at considerable speed. That is to say, the fibrous slurry is continuously deposited onto the forming fabric in the forming section, while a newly manufactured paper sheet is continuously wound onto rolls after it exits from the dryer section at the downstream end of the paper machine.
Referring, now, more specifically to the dryer section, in the dryer section, the dryer cylinders may be arranged in a top and a bottom row or tier. Those in the bottom tier are staggered relative to those in the top tier, rather than being in a strict vertical relationship. As the sheet proceeds through the dryer section, it passes alternately between the top and bottom tiers as it passes first around a dryer cylinder in one of the two tiers, then around a dryer cylinder in the other tier, and so on sequentially through the dryer section.
The top and bottom tiers of dryer cylinders may each be clothed with a separate dryer fabric. In such a situation, the paper sheet being dried passes unsupported across the space, or “pocket”, between each dryer cylinder and the next dryer cylinder on the other tier.
In a single tier dryer section, a single row of cylinders along with a number of turning cylinders or rolls may be used. The turning rolls may be solid or vented.
In order to increase production rates and to minimize disturbance to the sheet, single-run dryer sections are used to transport the sheet being dried at high speeds. In a single-run dryer section, a paper sheet is transported by use of a single dryer fabric which follows a serpentine path sequentially about the dryer cylinders in the top and bottom tiers.
It will be appreciated that, in a single-run dryer section, the dryer fabric holds the paper sheet being dried directly against the dryer cylinders in one of the two tiers, typically the top tier, but carries it around the dryer cylinders in the bottom tier. The fabric return run is above the top dryer cylinders. On the other hand, some single-run dryer sections have the opposite configuration in which the dryer fabric holds the paper sheet directly against the dryer cylinders in the bottom tier, but carries it around the top cylinders. In this case, the fabric return run is below the bottom tier of cylinders. In either case, a compression wedge is formed by air carried along by the backside surface of the moving dryer fabric in the narrowing space where the moving dryer fabric approaches a dryer cylinder. The resulting increase in air pressure in the compression wedge causes air to flow outwardly through the dryer fabric. This air flow, in turn, forces the paper sheet away from the surface of the dryer fabric, a phenomenon known as “drop off”. “Drop off” can reduce the quality of the paper product being manufactured by causing edge cracks. “Drop off” can also reduce machine efficiency if it leads to sheet breaks.
Many paper mills have addressed this problem by machining grooves into the dryer cylinders or rolls or by adding a vacuum source to those dryer rolls. Both of these expedients allow the air otherwise trapped in the compression wedge to be removed without passing through the dryer fabric, although both are expensive.
In this connection, fabric manufacturers have also employed application of coatings to fabrics to impart additional functionality to the fabric, such as “sheet restraint methods.” The importance of applying coatings as a method for adding this functionality to, for example, dryer fabrics, has been cited by Luciano-Fagerholm (U.S. Pat. No. 5,829,488 (Albany), titled, “Dryer Fabric With Hydrophilic Paper Contacting Surface”).
Luciano and Fagerholm have demonstrated the use of a hydrophilic surface treatment of fabrics to impart sheet-holding properties while maintaining close to the original permeability. However, this method of treating fabric surfaces, while successful in imparting sheet restraint, enhanced hydrophilicity and durability of the coating is desired. WO Patent 97/14846 also recognizes the importance of sheet restraint methods, and relates to using silicone coating materials to completely cover and impregnate a fabric, making it substantially impermeable. However, this significant reduction in permeability is unacceptable for dryer fabric applications. Sheet restraint is also discussed in U.S. Pat. No. 5,397,438, which relates to applying adhesives on lateral areas of fabrics to prevent paper shrinkage. Other related prior art includes U.S. Pat. No. 5,731,059, which reports using silicone sealant only on the fabric edge for high temperature and anti-raveling protection; and U.S. Pat. No. 5,787,602 which relates to applying resins to fabric knuckles. All of the above referenced patents are incorporated herein by reference.
The present invention is another approach toward a solution to this problem in the form of a dryer fabric having backside vents which permit air trapped in a compression wedge to escape without having to pass through the dryer fabric. The present invention also includes a method for manufacturing the dryer fabric.